Hacker News Reader

Comment by changoplatanero | Hacker News Reader

what does k-sortable mean?

That if a hundred servers are generating (uuid, timestamp) tuples that are subsequently merged on a single machine, and sorted by uuid, it would have almost the same order as if sorted by timestamp.

This property is useful for RDBMS writes, when the UUID is used as a primary key and this locality ensures that fewer slotted pages need to be modified to write the same amount of data.

Is that what they mean by "used as the primary key in a database while ensuring good locality"/"database locality"? That read/write access will hit fewer disk pages?

yes, exactly

> it would have almost the same order as if sorted by timestamp.

Is there documentation covering the scenarios on how the order can become out of sync and what the odds are? There's a big difference between "almost" and "always" if we're talking about using this as a database PK.

> There's a big difference between "almost" and "always"

Not in the context of an RDBMS, which use b+/b*-tree variants (or LSM sstables). Sequentially generated UUID's will end up near each other when sorted lexicographically, regardless of the fact that the sort order doesn't perfectly match the timestamp order.

The key seems to be based on UUIDv7, starting with a timestamp in milliseconds.

So the order can become out of sync if multiple events happen in the same millisecond; or if your servers' clock error is greater than a millisecond (i.e. if you're an NTP user)

More than sufficient for things like ordering tweets. If you're ordering bank account transactions, well, you'd probably be using transactions in an ACID-compliant relational database.

That it is a nearly sorted sequence. Typically the first part of the ID is a timestamp. So you may sort the IDs down to the second it was created. But for IDs created at the same time the order is random. This can be used for caching, database performance and so on.

Reply on news.ycombinator.com